The shadow business secretary said Ms Kendall was the candidate who could move the party “beyond our comfort zone”.
The endorsement from Mr Umunna - who dramatically withdrew from the contest earlier this month - came in an article for the New Statesman.
In a joint article with his ex-campaign team, he said Labour needed to tell a “credible national story of a country proud of its history and confident of owning the future”.
“A vision of a Britain in which all can get on, whose citizens are financially secure and in control of their lives and happiness - and are, collectively, secure and effective in the wider world,” he went on.
“For us, our next leader must get this vision right. On all these big subjects, Liz Kendall has asked the tough questions and started to chart a course to the answers. She has been courageous in challenging conventional wisdom.”
The backing comes as a huge boost to her campaign at a time when her two biggest rivals are pushing ahead.
Shadow health secretary Andy Burnham, widely regarded as the frontrunner, and shadow home secretary Yvette Cooper are both expected to secure well over the required 35 nominations.
Ms Kendall’s endorsement came as Wakefield leadership contender Mary Creagh told the party it is not going to win over voters by having a “Rubik’s Cube approach” to politics.
Asked how she would appeal to those Labour voters who went to Ukip, those who went to the Tories and those who went to the SNP, Ms Creagh told BBC Radio 4’s Woman’s Hour: “It’s a massive, massive challenge and we aren’t going to do it by sort of having a Rubik’s Cube approach to politics, where we put one face to one person and then try and present another face to another, because we just end up in a scramble.”
Her comments might be interpreted by some as a swipe at former Labour leader Ed Miliband.
Mr Miliband memorably told GQ magazine he was a “bit square” growing up and was “good at the Rubik’s Cube”.
Hours after speaking it emerged Mrs Creagh had won the support of new MP Stephen Kinnock, son of the former Labour leader.
Meanwhile Mrs Cooper has sought to move her campaign up a gear by arguing that Labour should offer free childcare for all.
A Scandinavian-style system of universal childcare would see 30 hours of free care for all pre-school youngsters over the age of two.
For younger children, there should be a new system of tax credits to cover the period after a mother finishes maternity leave, she said.
As party heavyweights continued to fight it out, more names were coming forward for the deputy leader post.
South Yorkshire MP John Healey has confirmed he is now in the running, with the backing of neighbouring MPs Sir Kevin Barron and Sarah Champion.
Mr Healey, who competitors for the job include Doncaster’s Caroline Flint, said: “I had no intention of running for Deputy Leader, but I’ve been dismayed at how narrow and shallow Labour’s debate has been so far.
“Labour suffered a terrible defeat at the election and we’ve got to prove that we’re up to the job.”